This invention relates to a mask pattern correction method applicable to a mask to be used in the lithography process of manufacturing semiconductor devices. It also relates to an exposure mask to be used for such a method.
The upward trend of integration and operating speed of semiconductor devices in recent years has entailed ever more rigorous requirements to be met for dimensionally controlling device patterns in the manufacture of semiconductor devices. Additionally, the problem of OPE (optical proximity effect) attributable to the manufacturing process has become remarkable as a result of down-sizing of devices.
In the manufacture of semiconductor devices, the conditions of the manufacturing process are tuned so as to make the areas with the least process margin show intended (designed) dimensions. Those areas are where their dimensions have to be selected very carefully and, in the case of a semiconductor memory, they may refer to the memory cells where the pattern elements show the highest density. However, if the conditions of the manufacturing process are adapted to the memory cells where the pattern elements shows the highest density, the peripheral circuits where the pattern elements are not very densely arranged can be affected by the optical proximity effect (OPE) attributable to the manufacturing process to show discrepancies between the designed dimensions and the actual dimensions. The optical proximity effect is in fact a phenomenon given rise to by a number of factors including the optical image of the pattern obtained after passing through an exposure mask, the latent image existing in the resist, the process of applying resist and developing the latent image, the make of the backing film, the make of the etching of the backing film, the postprocessing such as cleaning and oxidation and the process of preparing the exposure mask, which may affect each other to make the phenomenon a very complicated one.
Thus, the optical proximity effect is attributable not only to optical factors and a number of research laboratories have been paying efforts in the development of technologies on optical proximity correction (OPC). According to many of the published research papers on OPC, most OPC technologies are based on simulated optical images.
However, as pointed out above, there are a number of non-optical factors that are responsible for the OPE in the mask wafer process and, therefore, the OPE of the overall process on wafers has to be looked into in order to properly correct the dimensions of the masks used in the process.
Known techniques for correcting one-dimensional gate patterns realized by taking the overall mask wafer process into consideration include the buckets method (L. Liebman et al., SPIE, Vol. 2322, Photomask Technology and Management (1994), 229). The buckets method will be briefly discussed hereinafter by referring to FIGS. 1 through 3.
With the buckets method, a TEG (test element group) of finished wafers having measured sizes with an ACLV (across the chip linewidth variation) is used after the mask wafer process to electrically determine the relationship between the dimensional variation (difference between the designed size and the produced size) and the distance between the pattern under observation 10 and each of the directly adjacent patterns 12 (dependency on the distance to the nearest neighbor) (see FIG. 1).
Then, the dimensional variation per edge is determined on each wafer by using characteristic data as shown in FIG. 2. Then, the points that are electrically located on the grid of the mask writing system are picked up by dividing the obtained result by the smallest grid width (on the wafer) of the mask writing system, referring to a "0" variation point from the intended size. Subsequently, a correction region for 1 grid, a correction region for 2 grids and so on are determined by means of the x-coordinate values (a, b, c, . . . ) of the picked up points to prepare a mask correction rule as shown in FIG. 3.
With this technique, the dimensional difference per edge due to the variation in the pattern density can be theoretically reduced from .DELTA.1 to .DELTA.2.
However, the buckets method proposed by L. W. Liebman et al. for correcting a gate pattern is accompanied by the problems as will be discussed below by referring to FIGS. 4 and 5.
Problem 1
Referring to FIG. 4, as for a dimensional correction of a device, firstly, a pattern of the device is designed with predetermined dimensions in design process Sl. After passing through mask process S2 and lithography process S3, an actual pattern is prepared in etching process S4. Then, the dimensions of the pattern are electrically determined in measurement process S5 and a finished pattern is produced in on-wafer dimensions finalizing process S6.
With the known buckets correction method of L. W. Liebman et al., the mask correction rule is determined by assuming that the correlation (correction factor) between the dimensional variation of wafer and the amount of correction in process S6 is expressed by a linear function with a coefficient of "1". This means that, if a dimension on the wafer is greater than the corresponding dimension of the pattern by 50 nm, the dimension of the pattern is reduced by 50 nm.
However, as pointed out above, the OPE is affected by various factors (including the mask process, the lithography process, the etching process and so on in FIG. 4) and hence the processes in FIG. 4 do not necessarily show a linear relationship with a coefficient of "1". For example, in the lithography process 3, the coefficient will become greater than "1" and the linear relationship will no longer be held as the size of the pattern is reduced (as in the area left to the vertical broken line in FIG. 5).
A coefficient exceeding "1" refers to a state where a dimension on the wafer is greater than the corresponding dimension of the pattern by more than 50 nm but the dimension of the pattern will become too small if it is reduced by 50 nm by applying the known buckets correction method of L. W. Liebman et al. Thus, the known buckets correction method cannot be used to accurately correct the dimensions of a mask.
Problem 2
With the known buckets correction method of L. W. Liebman et al., the dimensional difference per edge between the finished pattern and the intended pattern is determined by the grid width of the mask writing system and equal to .DELTA.2 as shown in FIG. 2. The difference should be reduced further.
Problem 3
When the known buckets correction method is applied to mass production, there arises the following problem.
Generally, same devices may be manufactured in more than one factories or on more than one production lines. If they are manufactured on a same single line, more than manufacturing apparatus may be used for them. As a matter of course, the pattern density dependency of the dimensions of the patterns on the finished wafers may vary from factory to factory, from line to line and/or from apparatus to apparatus. If the pattern density dependency of the patterns on the finished wafers varies, the correction rule is changed so that different correction masks will be necessary for the different factories, the different production lines and/or the different manufacturing apparatus.
However, the use of different correction masks among different factories, different production lines and/or different manufacturing apparatus for manufacturing same devices will be cumbersome from the viewpoint of preparing and controlling masks and inefficient in terms of manufacturing devices. Thus, there is a demand for a correction method with which a same correction mask can be used among different factories, different production lines and/or different manufacturing apparatus for manufacturing same devices.